Epoxy resins have excellent mechanical properties and thermal properties, and therefore are widely used in various fields. Imidazole is used as a curing agent for curing such epoxy resins. But, a problem of an epoxy resin-imidazole mixed liquid is that it cannot be used as a one-component type because curing starts early and it is thickened or gelled in long-term storage.
Accordingly, the use of an imidazole acid addition salt in which hydroxybenzoic, acid is added to imidazole, as the curing agent (see Patent Document 1), and the use of a clathrate of a tetrakisphenol compound (for example, 1,1,2,2-tetrakis(4-hydroxyphenyl)ethane (hereinafter referred to as TEP)) and imidazole as the curing agent (see Patent Document 2) are proposed. Particularly, Patent Document 3 proposes an epoxy resin-based, organic solvent type paint using xylene for a clathrate of TEP and 2-ethyl-4-methylimidazole. This achieves a certain effect, but is not satisfactory yet. Particularly, storage stability with a polar solvent that is a good solvent is insufficient.
The present inventors propose a curable resin composition using a clathrate of an isophthalic acid compound and imidazole (see Patent Document 4). But, this does not describe storage stability or curability for use as a liquid curable epoxy composition.
In addition, Patent Document 5 describes a one-component epoxy resin composition containing a salt consisting of 2 moles of an imidazole compound having a long-chain alkyl group at position 2 and 1 mole of an aliphatic dicarboxylic acid, as a curing agent and a curing accelerator, which can also be cured in a low temperature range and also has excellent storage stability. But, the clathrate used in the invention has an aliphatic polyvalent carboxylic acid (host compound):guest compound inclusion ratio of 1:1, and Patent Document 5 describes the tensile shear strength of the cured product, but does not describe the organic solvent resistance of the cured product.
Patent Document 6 describes a liquid epoxy resin composition consisting of a salt of pyromellitic acid and an imidazole or an imidazoline. Patent Document 7 describes a liquid epoxy resin composition consisting of a salt of a hydroxyalkylated imidazole and an organic acid, particularly pyromellitic acid. But, although the cured products have excellent heat resistance or moisture resistance, Patent Documents 6 and 7 do not describe organic solvent resistance.